Php 无法查看从父类继承的属性

我用以下变量创建了一个银行账户类，

$Balance

变量是私有的，因此我使用setter和getter函数访问Balance的私有属性。

class BankAccount
{
    public $HoldersName;
    public $AccountNumber;
    public $SortCode;
    private $Balance = 1;
    public $APR = 0;
    public $Transactions = []; //array defined.

    public function __construct($HoldersName, $AccountNumber, $SortCode, $Balance = 1) //constructor where i.e. HoldersName is the same as the variable $HoldersName. 
    {                                                                                  //constructor automatically called when object is created.
        echo "Holders Name: " . $this->HoldersName  = $HoldersName . "</br>";
        echo "Account Number: " . $this->AccountNumber = $AccountNumber . "</br>";
        echo "Sort Code: " . $this->SortCode = $SortCode . "</br>";
        $this->Balance = $Balance >= 1 ? $Balance : 1;
    }


    public function set_Balance($Balance) //SET balance.
    {
        return $this->Balance=$Balance;
    }

    public function get_Balance() //GET balance.
    {
        return $this->Balance; //Allows us to access the prviate property of $Balance.
    }

}

在这里，我创建了类

SavingsAccount

的一个实例，余额为6800，我试图调用函数

SavingsAccount:：CalculateInterest（）

，但出现了以下错误：

请注意，第42行未定义的属性：SavingsAccount:：$Balance

//定义对象
$person2=新储蓄账户（'Peter Bond'，987654321'，11-11-11'，6800）//创建类SavingsAccount的实例
呼应“兴趣：”；
打印（$person2->CalculateInterest（））；

正如您所说，

$Balance

属性是私有的。Private意味着它不能从定义类外部访问，甚至不能从继承属性的子类访问

<?php
class Foo {
    private $bar="baz";
    public function __construct(){echo $this->bar;}
    public function getBar(){return $this->bar;}
}
class Bar extends Foo {
    public function __construct(){echo $this->bar;}
}
new Foo;
new Bar;

所以，你有两个选择。您可以改用getter：

<?php
class Foo {
    private $bar="baz";
    public function __construct(){echo $this->bar;}
    public function getBar(){return $this->bar;}
}
class Bar extends Foo {
    public function __construct(){echo $this->getBar();}
}
new Foo;
new Bar;

这两种方法中的任何一种都可以提供预期的输出：

baz
baz

有关属性和方法可见性的详细信息，请参见

baz
PHP Notice:  Undefined property: Bar::$bar in php shell code on line 2

<?php
class Foo {
    private $bar="baz";
    public function __construct(){echo $this->bar;}
    public function getBar(){return $this->bar;}
}
class Bar extends Foo {
    public function __construct(){echo $this->getBar();}
}
new Foo;
new Bar;

<?php
class Foo {
    protected $bar="baz";
    public function __construct(){echo $this->bar;}
    public function getBar(){return $this->bar;}
}
class Bar extends Foo {
    public function __construct(){echo $this->bar;}
}
new Foo;
new Bar;

baz
baz